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“C Language Beginner’s Essential Knowledge Notes Series of 100 Articles”“

26. Summary of switch-case Statement Usage Techniques: Multi-branch Selection

1. Basic Structure of switch Statement

The switch statement provides multi-branch selection functionality, which is clearer than multiple if-else statements:

switch (expression) {
    case constant1:
        statement1;
        break;
    case constant2:
        statement2;
        break;
    default:
        default_statement;
}

Execution Flow

1. 1. Calculate the value of the expression
2. 2. Match with each case constant
3. 3. Execute the statement of the matching case
4. 4. Exit upon encountering break or the end of switch

2. Basic Usage Examples

1. Simple Branch

int day = 3;
switch (day) {
    case 1: printf("Monday"); break;
    case 2: printf("Tuesday"); break;
    case 3: printf("Wednesday"); break;
    default: printf("Invalid input");
}

2. Character Judgment

char grade = 'B';
switch (grade) {
    case 'A': printf("Excellent"); break;
    case 'B': printf("Good"); break;
    case 'C': printf("Pass"); break;
    default: printf("Fail");
}

3. Key Syntax Features

1. Expression Limitations

• • Must be of integer or enumeration type (int, char, enum, etc.)
• • Cannot be floating-point or string

2. Case Constant Requirements

• • Must be a compile-time constant expression
• • Duplicate case values are not allowed

3. Importance of break

int x = 1;
switch (x) {
    case 1: printf("1");  // No break, continues to execute case 2
    case 2: printf("2");  // Outputs "12"
}

4. Advanced Usage Techniques

1. Multiple cases sharing code

int month = 2;
switch (month) {
    case 1: case 3: case 5: case 7: case 8: case 10: case 12:
        printf("31 days"); break;
    case 4: case 6: case 9: case 11:
        printf("30 days"); break;
    case 2:
        printf("28 or 29 days"); break;
}

2. Using Enumeration Types

enum Color {RED, GREEN, BLUE};
enum Color c = GREEN;
switch (c) {
    case RED: printf("Red"); break;
    case GREEN: printf("Green"); break;
    case BLUE: printf("Blue"); break;
}

3. Range Judgment Techniques

int score = 85;
switch (score / 10) {
    case 10: case 9: printf("A"); break;
    case 8: printf("B"); break;
    case 7: printf("C"); break;
    default: printf("D");
}

5. Comparison with if-else

Feature	switch-case	if-else
Applicable Scenarios	Discrete value multi-branch	Range judgment or complex conditions
Execution Efficiency	Usually higher (jump table)	Linear judgment
Readability	Clear branches	Difficult to read when nested complex
Type Limitations	Only integer/enumeration	Any expression

6. Common Errors

1. Forgetting break

switch (x) {
    case 1: printf("1");
    case 2: printf("2");  // When x=1, will output "12"
}

Correct approach:

switch (x) {
    case 1: printf("1"); break;
    case 2: printf("2"); break;
}

2. Variable case values

int y = 2;
switch (x) {
    case y: ...  // Error! case must be a constant
}

Correct approach:

#define Y_VALUE 2
switch (x) {
    case Y_VALUE: ...
}

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